The rapid expansion in the use of the Internet has resulted in a demand for high speed communications links and devices, including optical links and devices. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Optoelectronic integrated circuits made of silicon are highly desirable since they could be fabricated in the same foundries used to make VLSI integrated circuits. Optical communications technology is typically operating in the 1.3 μm and 1.55 μm infrared wavelength bands. The optical properties of silicon are well suited for the transmission of optical signals, due to its transparency in the infrared wavelength bands of 1.3 μm and 1.55 μm and its high refractive index. As a result, low loss planar silicon optical waveguides have been successfully built.
A silicon based waveguide is just one of many components needed to make an integrated optoelectronic circuit. An optical signal received by an optoelectronic circuit has in many cases to be converted to an electronic signal for further processing by electronic circuits. Conversion of optical signals to electronic signals can be achieved by a photodiode. Silicon, due to its bandgap of 1.12 eV, cannot be used to make photodiodes for infrared band signals, because it is transparent to light at wavelengths above 1.1 μm. Silicon's transparency to infrared light makes it ideal for use as a planar waveguide on an integrated circuit, but eliminates it from use as an infrared photodiode. However, germanium can be used in an infrared diode and may be incorporated into a germanium on silicon integrated waveguide photodiode. However, in many cases, large optical losses due to scattering may be incurred when light is transmitted from the silicon layer to the germanium layer of a germanium on silicon integrated waveguide photodiode, leading to signal reduction.
For some applications, it would be advantageous to optimize various characteristics of photodiode devices. Examples of important characteristics include, but are not limited to, the capacitance of the photodiode device, the dark current of the waveguide photodiode devices and/or system noise. In some cases, reducing the length and/or footprint of the photodiode may improve these characteristics; however, for many current photodiode system designs, reducing photodiode length and/or footprint may result in poor responsivity and/or loss of sensitivity. Mechanisms for improving the coupling of light into the photodiode device would be useful for addressing some of these issues.
Reducing the length of a waveguide photodiode could reduce its capacitance. Similarly, reducing the footprint of a photodiode could also reduce the dark current of the waveguide photodiode, possibly enabling lower noise operation and higher device density. However, many current photodiode designs lose responsivity and/or sensitivity when their size and/or length are reduced.
For many applications it would be beneficial to minimize the capacitance of the germanium on silicon waveguide photodiode. For example, in some cases, a transimpedance amplifier with a larger feedback resistor may be used to amplify the photocurrent in a reduced capacitance waveguide photodiode; in this configuration, better noise performance may be achieved while maintaining the overall bandwidth of the waveguide photodiode plus transimpedance amplifier subsystem. If it were possible to reduce the capacitance of a waveguide photodiode, the power consumption of the transimpedance amplifier could possibly be reduced. A reduced capacitance photodiode system might enable the use of other non-conventional transimpedance architectures.
Optical loss mechanisms limit the maximum fraction of the light that can be absorbed by a germanium on silicon waveguide photodiode. Examples of these loss mechanisms include, but are not limited to, absorption and scattering by the electrodes, optical leakage of the waveguide photodiode modes and surface roughness of the germanium stripe. These optical loss mechanisms compete with the absorption by the germanium. It would be advantageous to reduce the effect of these optical loss mechanisms by improving the overlap of light with the germanium layer.
In some cases, a mode matching mechanism such as an adiabatic taper defined in the germanium layer may be used to increase the overlap of the light with the germanium when light is injected from a silicon waveguide to the germanium on silicon waveguide photodiode. The use of an adiabatic taper of the germanium layer may result in a device with adequate responsivity, but can introduce limitations such as, but not limited to, increased device capacitance and device footprint. Furthermore, some features of an adiabatic taper can be difficult to manufacture such as, but not limited to, the very narrow and sharp feature at the beginning of an adiabatic taper; manufacturing difficulties may be due, at least in part, to limited lithography resolution and/or to limitations associated with selective germanium epitaxy processes and/or selective germanium removal processes. In some cases, these constraints may be captured by design rules that limit minimum dimensions and exclude certain geometries from design. Breaking these design rules can result in poor yields and/or unmanufacturable devices. What is needed is a way to design and implement a germanium on silicon integrated waveguide photodiode in a way that provides adequate sensitivity and responsivity with an acceptable device capacitance and a manufacturable geometry. Furthermore, device density considerations may make the use of adiabatic tapers unattractive for some applications; a more compact, but functional integrated silicon/silicon and germanium waveguide photodetector would be attractive for some applications.
Furthermore, other improvements to germanium on silicon integrated waveguide photodiode would be valuable such as, but not limited to, reducing optical losses from other sources such as scattering or absorption related to the electrical contacts on the germanium layer and lateral light leakage in the silicon layer.